You can leave the dump valves open all the time. You must use plenty of water to flush the solid wastes Fill the commode twice, flush twice. All freezing weather campers do it when their waste tanks or valves are susceptible to freezing.( did it for years) The lightwall pvc mentioned before is sdr 35, a sch. 20. grade it 1-1 1/2" in 10' Do not create a trap.
Septic tanks and absorption fields are a matter I do know a great deal about, with 4 commercial EPA licensed systems we designed and built, and having installed dozens for private homes. I cannot advise you on the system in place, not knowing its size, date or maintenance. Go to the local planning office to learn of any issues about your particular system. Wastewater is not a dump and forget issue. It does not disappear.
To hook up, dump into the septic tank if it has an inlet inspection riser, use it, thats easiest. Be certaain its the inlet not the outlet inspection riser. Next is through the big clean out lid somehow. not easy if its concrete or below grade. Best is through a cleanout upstream of tank, even if you have to excavate and install one. That might also shorten your drain line if it runs nearer your rig. Whether the tank has inspection risers is dependent on its construction date, local planning.